Bio-Oil Characteristics
Fast pyrolysis is a process that can produce pyrolysis oil, often termed bio-oil, from any lignocellulosic biomass type. Bio-oil is produced by fast pyrolysis of small biomass particles at 400° C. to 650° C. in the absence of oxygen. The yield of bio-oil is relatively high at 60% to 80% dry weight basis. Bio-oil chemical properties vary with the feedstock, but woody biomass typically produces a mixture of 30% water, 30% phenolics, 20% aldehydes and ketones, 15% alcohols and 10% miscellaneous compounds.
As a fuel, bio-oil has potential environmental advantages because combusted bio-oil produces half the NOX and no SOX when compared to petroleum fuels. Because bio-oil is produced from renewable biomass, it is considered to be CO2 neutral. Bio-oil can be burned directly in engines. Electricity has been produced by bio-oil fueled diesel engines, and turbines have been specially modified to successfully burn bio-oil. However, some properties of bio-oil such as lower octane, acidity, immiscibility with hydrocarbons, viscosity change over time and a distinctive odor have prevented its commercialization as a fuel to date, other than for use in pilot and demonstration projects. Researchers have unanimously concluded that some form of upgrading of raw bio-oils is required prior to utilization for fuel.
Bio-oil can contain up to 45 percent oxygen which is responsible for most of the various negative properties described. Bio-oil catalytic hydrodeoxygenation (HDO) has been investigated using various catalysts and has resulted in reducing or eliminating some of the negative properties of bio-oils due to the elimination of oxygen and/or reduction of double bonds and aldehyde and keto groups. Several groups of researchers are utilizing various catalysts to produce HDO bio-oil. However, approximately 3 wt % of hydrogen is required to apply HDO and nearly 30% of energy contained in the original raw bio-oil is lost in the process. Another potential upgrading process, referred to herein as esterification for the purpose of this patent, provides considerably higher yields and does not require the expense of hydrogen and the capital outlay for hydrotreating infrastructure.
Bio-Oil Upgrading Via Alcohols Addition
Bio-oil primarily contains reactive hydroxyl, carbonyl and carboxylic groups which undergo etherification and esterification reactions during storage. These condensation reactions generally increase the molecular weight and are responsible for bio-oil viscosity increase over time that makes raw bio-oil difficult to store. The problem of polymerization of the bio-oil can be minimized if these reactions are controlled. Researchers have demonstrated that these polymerization reactions can be slowed by reacting alcohols with the reactive sites of the oligomers present in the bio-oil. These alcohols have been shown to restrict the continuation of the polymerization reaction to increase bio-oil stability. The monofunctional alcohols can react with the acidic compounds present in the bio-oil to form polyesters which may again undergo esterification reactions with alcohol to form low molecular weight materials. Similarly, aldehydes and ketones can be changed to their corresponding acetals and ketals by reacting with alcohol, and this reaction will act to block the reactive sites and restrict the polymerization reactions which are responsible for the temporal increase in bio-oil viscosity (Diebold and Czernik 1997).
Further research was performed in 2000 (Boucher et al.) to determine the influence of alcohol's addition to stabilize and upgrade bio-oil produced from tree bark feedstock. The upgrading was practiced to produce a potential fuel for gas turbines. The methanol addition improved some properties of the raw bio-oil. Viscosity was reduced, higher heating value (HHV) was increased significantly to 32 MJ/kg, and accelerated aging was significantly reduced. The improvement of properties resulting from methanol addition correlate to those found by other investigators who have added alcohols to stabilize or improve the fuel value of raw bio-oils.
Esterification Via Alcohol and Catalyst Addition
“Esterification” will be referenced in this disclosure as the catalyzed reaction of the bio-oil organic acids and other compounds with alcohols. Reactions, such as etherification, acetalization, ketalization and others are known to occur as a result of the catalyzed esterification of alcohols combined with bio-oil. The term esterification, as used herein, will be utilized to simplify the description of our invention, but it is understood that all reactions resulting from a combination of alcohol, bio-oil and an esterifying catalyst are being referenced with the term “esterification” as applied in this disclosure. The chemical combinations of alcohol and acid catalyst to produce an esterification reaction with the carboxylic acids prevalent in bio-oil have been shown to decrease bio-oil acidity, as well as to convert aldehydes to acetals. While the esterification reaction has been shown to improve most bio-oil properties, the reaction also produces a significant amount of water from the reaction (Tang et al., 2009; Xiong et al., 2009; Peng et al., 2008; Peng et al., 2009; Tang et al., 2008; Deng et al., 2009; Zhang et al., 2006). This increased water acts to reduce the total energy yield from addition of alcohol and from the higher heating value provided by the esters, acetals and ketals resulting from the reaction.
Radlein et al. (1995, 1997) provided an early esterification reaction example by dissolving freshly produced pyrolytic tar in 51.6 wt % of ethanol. The water content of this high viscosity bio-oil was 1.74%. Therefore, the organic fraction of the tar was 46.7%. Molecular sieve media and 1.4 wt % of sulfuric acid comprised the reacted mixture. The mixture was kept at room temperature for a few hours. The formation of acetals and esters were detected by GC/MS analysis within a few minutes of the mixing. After two hours' reaction time, the amount of ethanol and water had both decreased significantly. The water content was 0.28%, whereas the amount of ethanol remaining in the mixture was 35.9 wt %. Higher amounts of ethyl acetate, acetyl formate and diethoxyacetal of hydroxyacetaldehyde were detected. All of these esterification reactions produced water in the reaction medium. However, the lower amount of water produced by this experiment indicated that the molecular sieve media removed significant amounts of both the water resulting from condensation reactions and a portion of the water initially present in the raw bio-oil.
Moens and Czernik (2008) performed esterification experiments with a stoichiometric, or excess, amount of alcohol. The product was a two-phase compound. One phase (39 wt %) was a distillate containing water, acids and other carbonyl groups. The upgraded phase (63 wt %) was a semi-solid with low acidity and reduced oxygen with a water content of zero. This route to esterified bio-oil appears to provide low yield and a product of such high viscosity that it is unusable as a heating fuel.
Hitten et al. (2009) published results of ethanol injection into a slow pyrolysis reactor. These practitioners attached the cross pipe carrying the pyrolysis vapors from the main pyrolyzer reactor tube to the condenser train. While the cross pipe leading to the reaction vessel was heated to 450° C., the reaction vessel itself was not heated. For this reason, the pyrolysis vapors inside the reaction vessel partially condensed in the reaction vessel such that the alcohol spray introduced into the reaction vessel necessarily contacted condensed vapors in liquid bio-oil form, as well as reacting with the pyrolysis vapors. Therefore, the Hitten et al. device, developed for application to a fast pyrolysis reactor, differs from the novel device and method of our invention in that, by our method, alcohol is injected into the hot vapor stream well prior to condensation of pyrolysis vapors.
Xiong et al. (2008) esterified bio-oil organic acids by application of a dicatopmoc ionic liquid (C6(mim)2-HSO4) rather than the traditional acid catalyst. A two-phase liquid was obtained which was separated to provide a 49% yield of upgraded product. The properties of the upgraded fraction were improved to provide an HHV of 24.6 MJ/kg, an increased pH to 5.1 from 2.9, and a reduction of water content to 8.2% from 29.8%. GC/MS analysis confirmed esterification of organic acids to esters. The loss of 51% of the total reaction products in the form of a discarded aqueous fraction, however, will likely hinder any commercialization of this process.
Therefore, past practitioners have upgraded bio-oil by combining alcohols with bio-oils or by catalyzing bio-oil/alcohol mixtures to produce esterification reactions. The esterification reactions have all been described as increasing bio-oil water content to some degree. This water has usually been left in the final esterified bio-oil, but, in this case, it acts to reduce total HHV and partially reduces the energy increase obtained from production of esters, acetals and ketals. Alternatively, water removal has been achieved by discarding any aqueous fraction that may be produced simultaneously with the upgraded fraction; utilization of a water adsorbent to remove water produced by esterification has also been practiced.
Thermochemolysis with Tetramethyl Ammonium Hydroxide
Tetramethyl ammonium hydroxide (TMAH) thermochemolysis is pyrolysis combined with methylation of the polar groups evolved from the degradation of bio-polymers. Reaction of TMAH in the presence of alcohol produces methyl derivatives by hydrolysis, as well as by methanolysis. TMAH thermochemolysis, in the presence of methanol, can also convert completely/partially phenols to their corresponding methyl derivatives (Challinor 2001).
TMAH thermochemolysis of phenolic and lignin model compounds containing a β-5 linkage provided the methylated stilbenes as major products (Kuroda et al. 2002). Another group of researchers used furaldehyde, benzaldehyde, hydroxybenzaldehyde, methoxybenzaldehyde, di-methoxybenzaldehyde and vanillin as model compounds for TMAH thermochemolysis and found that TMAH not only acts as the methylating agent, but also reacts with the aldehydes according to Cannizzaro reaction. An in-situ methylation of the reaction products formed the corresponding esters and ethers (Tanczos et al. 1997). The product distribution indicated that the pyrolytic process involved two sequential reactions. At first, the aldehydes form an equimolar amount of corresponding alcohols and carboxylic acids; in the second step, methylation occurs. TMAH is an excellent methylating agent at high temperature. Therefore, the benzyl alcohols are converted to their corresponding ether, and the tetramethylammonium salt of the carboxylic acid decomposes to the methyl ester.
For better understanding of the TMAH thermochemolysis process, synthetic and natural lignins were pyrolyzed in the presence of TMAH. Coniferyl alcohol dimethyl ether, erythro/threo-1-(3,4-dimethoxyphenyl)-1,2,3-trimethoxypropane, and pinoresinol dimethylether were identified from GC/MS analysis. Phenolic models and lignin end units having β-5 structure gave methylated stilbene products during the TMAH-thermochemolysis process (Kuroda et al. 2002 2006).
13C labeled TMAH was also used for the thermochemolysis study of synthetic lignin dimer model compounds. The mechanistic study showed a number of base catalyzed rearrangement and elimination reactions to the lignin dimer, resulting in the methylation of alcohols and phenols to methyl ethers (Filley et al. 1999).
The comparison between the compounds released by Py-GC/MS with and without the application of TMAH of non-woody lignin samples was performed by a group of researchers (Del Rio et al. 2007). Pyrolysis of lignin in presence of TMAH undergoes β-O-4 ether bond cleavage and forms methylated aldehydes, ketones and acids. Presence of TMAH also initiated the high-temperature saponification of esters of p-hydroxycinnamic acid. Cleavage of the ether linkage at C-4 resulted in the methylation of free carboxyl and hydroxyl groups.
The TMAH thermochemolysis of levoglucosan and levoglucosan acetate formed the methyl ester of saccharinic acid, methyl acetate, methyl levoglucosan and other compounds. The thermochemolysis of cellulose produced methyl esters of pentanoic acid and butanoic acid (Schwarzinger et al. 2002). Pyrolysis of cellulose and lignin in the presence of TMAH produced a significant amount of aromatic acid and fatty acid methyl esters (Gauthier et al., 2003). The TMAH thermochemolysis fully converted the model aliphatic acid to its corresponding methyl esters (Joll et al., 2002). TMAH thermochemolysis of lignin produced some completely methylated compounds, such as 1,2-dimethoxybenzene, 3,4-dimethoxytoluene, 4-ethyl-1,2-dimethoxybenzene, 1,2,4-trimethoxybenzene, 1,2-dimethoxy-4-(2-propenyl)benzene, 3,4-dimethoxybenzoic acid, methyl ester and others (Klingberg et al, 2005).
As described, TMAH thermolysis has been employed to methylate various organic compounds and to assist in identifying the chemical structures of lignins and other biopolymers. TMAH thermochemolysis to convert bio-oil acids and their corresponding esters and ethers has not been performed by past practitioners.
Fractionation of Raw Bio-Oil to Produce a Pyroligneous Fraction Prior to Esterification or Olefination
Marker and Petri (2008) disclose a method by which water fractionation is applied to obtain the pyroligneous and aqueous fractions as previously described. These inventors propose production of gasoline or diesel hydrocarbons from the pyroligneous fraction by two stages: hydrotreating followed by hydrocracking. It is proposed that the aqueous fraction be reformed to hydrogen which is then input to the hydrotreating and hydrocracking process. The fractionation into pyroligneous and aqueous fractions may be performed by any method, but water fractionation is the preferred method. The pyroligneous fraction produced by this process is highly viscous, and processing this thick bio-oil fraction in hydrotreating reactors is expected to be problematic. Marker and Petri (2008) did not propose esterification, esterification followed by olefination, or simultaneous olefination/esterification to reduce the viscosity of the pyroligneous fraction to allow practical hydrotreating.
Olefination of Esterified Bio-Oil or Simultaneous Olefination/Esterification of Bio-Oil
The olefination of bio-oil model compounds and bio-oil has been attempted with limited success due to the immiscibility of olefin hydrocarbons with the water emulsion that comprises bio-oil (Yang et al. 2010; Zhang et al 2010). However, upgrading of bio-oil by olefinating esterified bio-oil, or by simultaneous esterification and olefination of bio-oil, combined with alcohol has not been reported.
As discussed previously, the esterification of bio-oil produces an upgraded product with improved properties. However, water content is increased significantly. Suggested methods to remove this water include utilization of molecular sieves as adsorbents. This method is likely to be prohibitively expensive, and recycling or disposal of utilized molecular sieve material may be problematic.